I think these cards have been over nerfed in recents updates.
I agree Ultimate Seal was too powerful when the damage was 100% of the attack but 50% is not enough. I mean the damage is now comparable to Pit Trap which is only a gold card. The same goes for Abyss and CD being 6 I feel it is less effective than Pit Trap.
These 2 cards don't have a Legend level right now.
To get them back their legendary status I suggest to put Ultimate Seal at 75%, and Abyss at 30% or decrease its CD down to 3 so it can be really effective in the early stage of a battle.
Ultimate Seal & Abyss
Re: Ultimate Seal & Abyss
There is some validity here. I seem to be the only one using ultimate seal anymore and that is only because it is level 40. It stops an Aphrodite whereas a pit trap doesn't. I also use it against epic cards like godfather and legends like red dragon and it does a little better than a maxed pit trap. But truthfully I do find myself using pit trap equally as much. There are not many players at the top using traps anymore bc they have fallen out of favor due to being much weaker now. Before they were too powerful at 100% for level 1 everyone used it. Now it's got to be at max level to be that good otherwise at 50 or even 60% it's not worth using at all and unlikely to be considered for its level up value. A good suggestion above - starting it at 75% and maybe going up in 1/2% increments to 100% at level 50. I could even see this card being considers for 75-125% range. Perhaps you should have goce beta test this some more. 
Abyss. Yes this card needs serious help. I've used it personally on just a few occasions. Sometimes to stop a magister that a low level player has acquired and I've seen it used against me as the preferred trap against a dragon slayer. I am assuming the thought process is that this card is a one for one? It stops one of their cards completely and after five rounds both cards die. However in many cases my units have destroyed the eligible targets only to have this card sitting uselessly bc all other targets remaining are either hell legion are low hp. So even though it is theoretically a one for one in practical terms it often doesn't end up that way.
Abyss. Yes this card needs serious help. I've used it personally on just a few occasions. Sometimes to stop a magister that a low level player has acquired and I've seen it used against me as the preferred trap against a dragon slayer. I am assuming the thought process is that this card is a one for one? It stops one of their cards completely and after five rounds both cards die. However in many cases my units have destroyed the eligible targets only to have this card sitting uselessly bc all other targets remaining are either hell legion are low hp. So even though it is theoretically a one for one in practical terms it often doesn't end up that way.